High energy density nanocomposites and related methods of preparation

ABSTRACT

Disclosed are metal oxide-polymer composites having a substrate comprising a metal oxide component, an aluminum oxide component, and a metallocene olefin polymerization catalyst component coupled to the substrate. The compositions can be used as thin films in an insulator device. Also disclosed is a method of preparing such compositions.

This application claims priority benefit of provisional application Ser.No. 60/859,873 filed on Nov. 17, 2006, the entirety of which isincorporated herein by reference.

The United States Government has certain rights to this inventionpursuant to Grant No. N00014-05-1-0766 from the Office of Navel Researchto Northwestern University.

BACKGROUND OF THE INVENTION

Future pulsed-power and power electronic capacitors will requiredielectric materials ultimately having energy storage densities >30J/cm³, with operating voltages >10 kV, and msec-μsec charge/dischargetimes with reliable operation near the dielectric breakdown limit.Importantly, at 2 J/cm³ and 0.2 J/cm³, respectively, the operatingcharacteristics of current state-of-the-art pulsed power and powerelectronic capacitors, which utilize either ceramics or polymers asdielectric materials, remain significantly short of this goal. An orderof magnitude improvement in energy density will require development ofrevolutionary new materials that substantially increase intrinsicdielectric energy densities while operating reliably near the dielectricbreakdown limit. For simple linear response dielectric materials, energydensity is defined in eq. 1, where ∈_(r) is relative dielectricpermittivity, E is the dielectric breakdown strength, and ∈₀ is thevacuum permittivity. Generally, inorganic metal oxides exhibit highpermittivities, however, they suffer from low breakdown fields. Whileorganic materials (e.g., polymers) can provide high breakdown strengths,their generally low permittivities have limited their application.U _(e)=½∈_(r)∈₀ E ²  (1)

Recently, inorganic-polymer nanocomposite materials have attracted greatinterest due to their potential for high energy density. By integratingthe complementary properties of their constituents, such materials cansimultaneously derive high permittivity from the inorganic inclusionsand high breakdown strength, mechanical flexibility, facileprocessability, light weight, and properties tunability (molecularweight, comonomer incorporation, thermal properties, etc.) from thepolymer host matrix. Additionally, there are good reasons to believethat the large inclusion-matrix interfacial areas will afford higherpolarization levels, dielectric response, and breakdown strength.

Although inorganic-polymer nanocomposites can be prepared via mechanicalblending, solution mixing, in situ radical polymerization, and in situnanoparticle synthesis, host-guest incompatibilities frequently resultin nanoparticle aggregation and phase separation, detrimental to theelectrical properties. Covalently grafting polymer chains to inorganicnanoparticle surfaces has also proven promising, leading to moreeffective dispersion and enhanced properties, however, such processesmay not be cost-effective and nor easily scaled up.

Illustrating another approach, in the large-scale heterogeneous orslurry olefin polymerizations practiced on a huge industrial scale, SiO₂is generally used as the catalyst support. However, very large localhydraulic pressures arising from the growing polyolefin chains are knownto effect extensive SiO₂ particle fracture and lead to SiO₂-polyolefincomposites. As a result, there remains an on-going search in the art foran alternate route to inorganic-polymer nanocomposites, to betterutilize the benefits and advantages afforded by such materials.

SUMMARY OF THE INVENTION

In light of the foregoing, it is an object of the present invention toprovide various high energy nanocomposites, related components anddevices, and/or methods for their preparation and/or assembly, therebyovercoming various deficiencies and shortcomings of the prior art,including those outlined above. It will be understood by those skilledin the art that one or more aspects of this invention can meet certainobjectives, while one or more other aspects can meet certain otherobjectives. Each objective may not apply equally, in all of itsrespects, to every aspect of this invention. As such, the followingobjects can be viewed in the alternative with respect to any one aspectof this invention.

It can be an object of the present invention to provide one or moremethods for nanocomposite preparation to prevent nanoparticleagglomeration problems associated with the prior art.

It can be another object of the present invention to provide an in situpolymerization technique using one or more metallocene catalystcomponents supported on a nanoparticle, with a range of available olefinmonomers.

It can be another object of the present invention, alone or inconjunction with one of the preceding objectives, to provide ananocomposite comprising a nanoparticle component homogeneouslydispersed within a matrix of a high-strength, high-energy commoditypolymer material of the sort used in the art with energy storagecapacitors and insulators.

Other objects, features, benefits and advantages of the presentinvention will be apparent from this summary and the followingdescriptions of certain embodiments, and will be readily apparent tothose skilled in the art having knowledge of various high energynanocomposites and assembly/production techniques. Such objects,features, benefits and advantages will be apparent from the above astaken into conjunction with the accompanying examples, data, figures andall reasonable inferences to be drawn therefrom, alone or withconsideration of the references incorporated herein.

In part, the present application can be directed to a particulatecomposition comprising a substrate comprising a metal oxide componentand an aluminum oxide component; and a metallocene olefin polymerizationcatalyst component coupled to such a substrate. Without limitation, sucha substrate and/or particulate can be nano-dimensioned. In certain otherembodiments, such a substrate and/or particulate composition can bemicro-dimensioned.

In certain embodiments, a metal oxide can be but is not limited tobinary and ternary metal oxides, such oxides as can comprise a dopant,and combinations thereof. In certain such embodiments, a metal oxidecomponent can be selected from Al₂O₃, SiO₂, TiO₂, ZrO₂, BaTiO₃BaZrO₃,PbO₃, together with Pb(TiZr)O₃ and other such oxides comprising adopant. Regardless, a metallocene component can be selected from anysuch polymerization catalyst known to those skilled in the art, madeaware of this invention. Without limitation, such a metallocenecomponent can be selected from EBIZrCl₂, CGCTiCl₂ and CpTiCl₃, asdescribed more fully below, and structural variations thereof. Withoutlimitation as to metal oxide and metallocene identity, such acomposition can be provided in a polyolefin matrix.

In part, the present invention can also be directed to a compositecomprising a nano-dimensioned substrate comprising a metal oxidecomponent, an aluminum oxide component and a metallocene catalystcomponent; and a polyolefin component coupled thereto. In certainembodiments, a polyolefin component can be selected from C₂ to about C₁₂polyalkylenes, substituted C₂ to about C₁₂ polyalkylenes, and copolymersthereof, such polyolefin components limited only by alkylene monomer (s)reactive with such a metallocene catalyst component under polymerizationconditions of the sort described herein. In certain such embodiments,metallocene and metal oxide components can be as described above orillustrated elsewhere herein. Accordingly, with choice of alkylenemonomer(s), such a polyolefin component can be select from isotacticpolypropylene, a linear polyethylene, and a polystyrene and copolymersthereof.

In part, the present invention can also be directed to a commodity orbulk material composition comprising a polyolefin component and anano-dimensioned or micro-dimensioned substrate component dispersedtherein, with such a substrate component comprising a metal oxidecomponent, an aluminum oxide component and metallocene catalystcomponent. Such metal oxide, aluminum oxide and metallocene componentscan be as described above. Without limitation as to substrate identity,volume fractions or percentages can range from about 0.05 percent toabout 15 percent. Likewise, substrate dispersion can be substantiallyhomogeneous on a nano- or microscale dimension. In certain suchembodiments, a metal oxide component of such a substrate can have ashape about or substantially spherical or a shape about or substantiallyrod-like, as demonstrated below.

A polyolefin component of such a composition is limited only by monomerpolymerization in the presence in such a metallocene catalyst. Forinstance, in certain embodiments, a polyolefin can be selected from C₂to about C₁₂ polyalkylenes, substituted C₂ to about C₁₂ polyalkylenes,and copolymers thereof. Regardless, depending upon polyolefin and/orsubstrate component identity, such a composition can be present as athin film and/or incorporated into a range of device structures,including but not limited to insulator devices. Alternatively, dependingupon a particular composition, such materials can find utility in thecontext of cable insulation.

In part, the present invention can also be directed to a method ofpreparing a metal oxide-polyolefin nanocomposite. Such a method cancomprise providing a substrate comprising a metal oxide component and ametallocene olefin polymerization catalyst component coupled thereto;and contacting such a substrate with an olefin component, such contactfor a time and/or an amount sufficient to at least partially polymerizean olefin on such a substrate, to provide a nanocomposite. Withoutlimitation, metal oxide, metallocene and/or olefin/alkylene componentscan be selected as described above. Depending upon olefin content anddegree of polymerization, such a substrate component can have a volumepercentage ranging from about 0.05 percent to about 15 percent. Incertain, embodiments, increasing volume percent can be used to affectmelt temperature, leak current density and/or relative permittivity of aresulting nanocomposite. In certain other embodiments, choice of metaloxide shape can be used to affect one or more composite physicalcharacteristics. Without limitation, the relative permittivity of such ananocomposite can be increased using a rod-shaped metal oxide component.

Illustrating yet another aspect thereof, the present invention can bedirected to a method of using an aluminoxane component to moderate phaseenergy densities of a metal oxide-polyolefin composite. Such a methodcan comprise providing a metal oxide component as can be selected frombinary and ternary metal oxides and such oxides comprising a dopant;contacting such a metal oxide component with an aluminoxane componentfor a time at least partially sufficient to provide an aluminum oxidecoating on the metal oxide component; contacting such a coated metaloxide with a metallocene olefin polymerization catalyst component, toprovide a nano- or micro-dimensioned substrate of the sort describedabove; and contacting such substrate with one or more olefin components,such contact for a time and/or an amount sufficient to at leastpartially polymerize the olefin(s) on such a substrate.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1. ¹³C NMR spectrum of an isotactic-polypropylene nanocomposite(100 MHz, C₂D₂Cl₄, 130° C.).

FIG. 2. ¹³C NMR spectrum of a poly(ethylene-co-1-octene) nanocomposite(100 MHz, C₂D₂Cl₄, 130° C.).

FIG. 3. ¹³C NMR spectrum of a syndiotactic-polystyrene nanocomposite(100 MHz, C₂D₂Cl₄, 130° C.).

FIGS. 4A-B. Electron microscopic characterization of: (A) as-receivedpristine ZrO₂ (SEM) and (B) 7.4 vol % ZrO₂-^(iso)PP nanocomposite (TEM).

FIGS. 5A-b. Electron microscopic characterization of: (A) as-receivedpristine TZ3Y (SEM) and (B) 31.1 wt % TZ3Y-^(iso)PP nanocomposite (TEM).

FIGS. 6A-B. Electron microscopic characterization of: (A) as-receivedpristine TZ8Y (SEM) and (B) 39.2 wt % TZ8Y-^(iso)PP nanocomposite (TEM).

FIG. 7. Representative C (capacitance) vs. A (electrode area) plot for a2.6 vol % BaTiO₃-^(iso)PP nanocomposite.

FIGS. 8A-C. Leakage current density vs. field measurement results forthe nanocomposite MIS or MIM devices (legends are for the volumefraction of the inorganic particles): (A) n⁺-Si/BaTiO₃-polypropylene/Au;(B) n⁺-Si/sphere-TiO₂-polypropylene/Au; (C)Al/rod-TiO₂-polypropylene/Au.

FIGS. 9A-C. Leakage current density vs. field measurement results forthe nanocomposite MIS or MIM devices (legends are for the volumefraction of the inorganic particles): (A) Al/ZrO₂-polypropylene/Au; (B)Al/TZ3Y-polypropylene/Au; (C) Al/TZ8Y-polypropylene/Au.

FIG. 10. Normalized effective permittivity (∈_(eff)−∈_(b)/∈_(a)−∈_(b))for composite dielectrics of polypropylene with spherical inclusions(eq. 4), and with ellipsoidal inclusions (eq. 5).

FIG. 11. Comparison of effective permittivities for spherical- androd-shaped TiO₂ nanoparticle-polypropylene nanocomposites.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

Illustrating certain embodiments of this invention, high energy densityBaTiO₃- and TiO₂-isotactic polypropylene nanocomposites were preparedvia in situ metallocene polymerization. The resulting nanocompositeswere found to have effective nanoparticle dispersion and to possessenergy densities as high as 9.4 J/cm³, as determined from relativepermittivities and dielectric breakdown measurements. To demonstratevarious other aspects of this invention, the scope of inorganicinclusion can be extended to include a broad variety of nanoparticles,with corresponding effects of nanoparticle identity and shape on theelectrical/dielectric properties of the resulting nanocomposites.Likewise, the scope of metallocene polymerization catalysts and olefinicmonomers can be extended (e.g., Chart 1) to enhance nanoparticleprocessability and thermal stability. Representative of a range such ofembodiments, nanoparticle coating with methylaluminoxane (MAO) andsubsequent in situ polymerization can be used effectively for effectivedispersion, to realize high breakdown strengths, permittivities andenergy storage densities.

Chart 1

Accordingly, a series of 0-3 metal oxide-polyolefin nanocomposites wassynthesized via in situ olefin polymerization using the metallocenecatalysts C₂-symmetric dichloro[rac-ethylenebisindenyl]zirconium (IV)(EBIZrCl₂), Me₂Si(^(t)BuN)(η⁵-C₅Me₄)TiCl₂(CGCTiCl₂), and(η⁵-C₅Me₅)TiCl₃(Cp*TiCl₃) immobilized on methylaluminoxane (MAO)-treatedbarium titanate (BaTiO₃), zirconium dioxide (ZrO₂), 3 mol %yttria-stabilized zirconia (TZ3Y), 8 mol % yttria-stabilized zirconia(TZ8Y), sphere-shaped titanium dioxide (TiO₂), and rod-shaped TiO₂nanoparticles. The resulting composite materials were characterized byX-ray diffraction (XRD), scanning electron microscopy (SEM),transmission electron microscopy (TEM), ¹³C nuclear magnetic resonance(NMR) spectroscopy, and differential scanning calorimetry (DSC). It wasshown by TEM that the nanoparticles are well-dispersed in the polymermatrix and each individual nanoparticle is surrounded by polymer.Electrical measurements reveal that most of the nanocomposites haveleakage current densities ˜10⁻⁸-10⁻⁶ A/cm², and the relativepermittivities of the nanocomposites increase as the nanoparticle volumefraction increases, with measured values as high as 6.1. At the samevolume fraction, rod-shaped TiO₂ nanoparticle-polypropylenenanocomposites exhibit greater relative permittivities than thecorresponding sphere-shaped TiO₂ nanoparticle-polypropylenenanocomposites. The energy densities of these nanocomposites areestimated to be as high as 9.4 J/cm³.

As one of the most commonly used polymers in large-scale powercapacitors, isotactic polypropylene offers greater stiffness, lowershrinkage, and less deterioration of the dielectric properties at highertemperatures than other grade polypropylenes. Therefore, theC₂-symmetric metallocene catalystdichloro[rac-ethylenebisindenyl]zirconium (IV) (EBIZrCl₂), known forhighly isospecific olefin polymerization, was selected to demonstrateimmobilization on the surfaces of MAO-treated metal oxide nanoparticles,to synthesize metal oxide-isotactic polypropylene nanocomposites.

X-ray diffraction (XRD) linewidth analyses using the Scherrer equationindicate that the microstructures and coherence lengths of theindividual nanoparticles remain largely unchanged upon deagglomerization(Table 1). (See, Jenkins, R.; Snyder, R. L. In Introduction to X-rayPowder Diffractometry; Winefordner, J. D., Ed.; Wiley: New York, 1996;pp 89-91; and Scherrer, P. Gött. Nachr. 1918, 2, 98-100.) ¹³C NMRspectroscopy (FIG. 1) shows that the present polypropylenes are highlyisotactic, as evidenced by the isotacticity index ([mmmm]=83%). (See,Busico, V.; Cipullo, R.; Monaco, G.; Vacatello, M. Macromolecules 1997,30, 6251-6263; Busico, V.; Cipullo, R.; Corradini, P.; Landriani, L.;Vacatello, M.; Segre, A. L. Macromolecules 1995, 28, 1887-1892; andZambelli, A.; Dorman, D. E.; Brewster, A. I. R.; Bovey, F. A.Macromolecules 1973, 6, 925-926.) DSC confirms the absence of extensiveamorphous regions in the composites since only isotactic polypropylenemelting features (142-147° C.) are detected. XRD data for thenanocomposites also reveal the presence of monoclinic a phasecrystalline isotactic polypropylene (2θ=14.2, 17.0, 18.6, and 21.8°). Itis found that the melting temperatures of the nanocomposites generallyincrease as the nanoparticle loading increases (Table 2), possibly dueto attractive interactions between the nanoparticles and the crystallineregions of the isotactic polypropylene.

TABLE 1 XRD Linewidth Analysis Results of the Nanocomposites 2θ FWHMCrystallite Size Powder (deg) (deg) (nm) BaTiO₃-polypropylene 31.6490.271 32.8 BaTiO₃ 31.412 0.254 35.6 TiO₂-polypropylene 25.358 0.361 23.5TiO₂ 25.360 0.317 27.1

Crystallite size (L) is calculated using the Scherrer equation L=0.9λ/Bcos θ_(B)(λ=x-ray wavelength, B=full-width-at-half maximum (FWHM) of thediffraction peak, and θ_(B)=Bragg angle).

Linear low-density polyethylene (LLDPE) is another polymer that iswidely used in power capacitors. Compared to isotactic polypropylene,the chain branching in the LLDPE affords better processability.Therefore, the sterically open constrained geometry catalystMe₂Si(^(t)BuN)(η⁵-C₅Me₄)TiCl₂(CGCTiCl₂) was utilized to synthesizeBaTiO₃-LLDPE nanocomposites via in situ ethylene+1-octenecopolymerization. FIG. 2 presents a representative ¹³C NMR spectrum ofthe nanocomposite, with the 1-octene incorporation level calculated tobe 25.0 mol %. (See, Qiu, X.; Redwine, D.; Gobbi, G.; Nuamthanom, A.;Rinaldi, P. L. Macromolecules 2007, 40, ASAP.) DSC measurements alsoconfirms the formation of LLDPE, which has a typical melting temperatureof 125.3° C.

Syndiotactic polystyrene has greater heat resistance than isotacticpolypropylene, which can only operate below 85° C. when incorporatedinto film capacitors. Employing the same protocol as EBIZrCl₂, thehalf-metallocene catalyst C_(P)*TiCl₃ ²⁵ was immobilized on MAO-treatedZrO₂ nanoparticles. Subsequent in situ styrene polymerization affordsZrO₂-syndiotactic polystyrene nanocomposites. A representative ¹³C NMRspectrum is shown in FIG. 3. The characteristic single resonance nearδ=145.6 ppm for the ipso phenyl carbon atom confirms the production ofsyndiotactic polystyrene, which is further substantiated by the meltingtemperature (267.0° C.) as measured by DSC.

During the course of in situ metallocene polymerization, the polymerchains propagating at the nanoparticle-immobilized metallocene catalyticcenters may be expected to create large local hydrostatic pressures andthus help to disrupt the nanoparticle agglomeration. Such results areconfirmed by the comparative electron microscopic characterization ofthe as-received pristine nanoparticles and the resulting nanocomposites.As can be seen from FIGS. 4, 5, and 6, the as-received pristinenanoparticles evidence very high levels of agglomeration, however, forthe polyolefin nanocomposites, the agglomeration of the nanoparticles isshown to be disrupted and each individual nanoparticle is surrounded bya layer of matrix polymer.

To assess nanocomposite permittivity properties, metal-insulator-metal(MIM) or metal-insulator-semiconductor (MIS) devices for nanocompositeelectrical measurements were fabricated by first doctor-bladingnanocomposite films onto aluminum or n⁺-Si substrates, followed byvacuum-depositing top gold electrodes through shadow masks. Thecapacitances were measured at 1 kHz, a sufficiently high frequency toavoid the complications arising from conduction and interfacialpolarization effects. After the capacitance was measured at multiplelocations on the nanocomposite film surface using different electrodeareas, the relative permittivity (∈_(r)) of the nanocomposite wasderived using eq. 2, where C is the capacitance, A is the electrodearea,

$\begin{matrix}{C = \frac{ɛ_{0}ɛ_{r}A}{d}} & (2)\end{matrix}$∈₀ is the vacuum permittivity (8.8542×10⁻¹² F/m), ∈_(r) is relativepermittivity, and d is the nanocomposite film thickness. FIG. 7 shows arepresentative capacitance vs. electrode area plot, the linearity ofwhich indicates the good dielectric uniformity of the nanocompositefilm.

Table 2 summarizes the relative permittivity measurement results for thepresent nanocomposites. As the nanoparticle loading increases, therelative permittivity of the nanocomposites also increases as predictedby the effective medium approximation. At the same volume fraction,rod-shaped TiO₂-polypropylene nanocomposites exhibit significantlygreater relative permittivities than those prepared with sphere-shapedTiO₂ nanoparticles (compare entries 1-4 versus 11-13) under identicalreaction conditions. Without limitation, this shape effect is thought toarise from the different depolarization factors for different inclusionparticle geometries.

TABLE 2 Electrical Characterization Results for MetalOxide-polypropylene Nanocomposites^(a) Film Nanoparticle T_(m) ^(c)Breakdown Thickness^(e) Energy Density^(f) Entry Composite vol %^(b) (°C.) Permittivity^(d) Field (kV) (μm) (J/cm³) 1 ^(iso)PP—^(s)TiO₂ 0.1%135.2 2.2 ± 0.1 >10.0 36 >0.8 ± 0.1  2 ^(iso)PP—^(s)TiO₂ 1.6% 142.4 2.8± 0.2 9.5 23 2.1 ± 0.2 3 ^(iso)PP—^(s)TiO₂ 3.1% 142.6 2.8 ± 0.1 7.5 271.0 ± 0.1 4 ^(iso)PP—^(s)TiO₂ 6.2% 144.8 3.0 ± 0.2 9.3 20 2.8 ± 0.2 5^(iso)PP—BaTiO₃ 0.5% 136.8 2.7 ± 0.1 8.8 28 1.2 ± 0.1 6 ^(iso)PP—BaTiO₃0.9% 142.8 3.1 ± 1.2 >10.0 21 >4.0 ± 0.6  7 ^(iso)PP—BaTiO₃ 2.6% 142.12.7 ± 0.2 9.8 25 1.8 ± 0.2 8 ^(iso)PP—BaTiO₃ 5.2% 145.6 2.9 ± 1.0 8.2 301.0 ± 0.3 9 ^(iso)PP—BaTiO₃ 6.7% 144.8 5.1 ± 1.7 9.0 22 3.7 ± 1.2 10^(iso)PP—BaTiO₃ 13.6% 144.8 6.1 ± 0.9 >10.0 17 >9.4 ± 1.3  11^(iso)PP—^(r)TiO₂ 1.4% 139.7 3.4 ± 0.3 12 ^(iso)PP—^(r)TiO₂ 3.2% 142.44.1 ± 0.7 13 ^(iso)PP—^(r)TiO₂ 5.1% 143.7 4.9 ± 0.4 14 ^(iso)PP—ZrO₂1.6% 142.9 1.7 ± 0.3 15 ^(iso)PP—ZrO₂ 3.9% 145.2 2.0 ± 0.4 16^(iso)PP—ZrO₂ 7.5% 144.9 4.8 ± 1.1 17 ^(iso)PP—ZrO₂ 9.4% 144.4 5.1 ± 1.318 ^(iso)PP-TZ3Y 1.1% 142.9 1.1 ± 0.1 19 ^(iso)PP-TZ3Y 3.1% 143.5 1.8 ±0.2 20 ^(iso)PP-TZ3Y 4.3% 143.8 2.0 ± 0.2 21 ^(iso)PP-TZ3Y 6.7% 144.92.7 ± 0.2 22 ^(iso)PP-TZ8Y 0.9% 142.9 1.4 ± 0.1 23 ^(iso)PP-TZ8Y 2.9%143.2 1.8 ± 0.1 24 ^(iso)PP-TZ8Y 3.8% 143.2 2.0 ± 0.2 25 ^(iso)PP-TZ8Y6.6% 146.2 2.4 ± 0.4 ^(a)Polymerizations carried out in 50 mL of tolueneunder 1.0 atm of propylene at 20° C. ^(b)From elemental analysis.^(c)From differential scanning calorimetry. ^(d)Derived from capacitancemeasurement. ^(e)Film thicknesses measured using profilometry ^(f)Energydensity (U) calculated from U = 0.5ε₀ε_(r)E_(b) ² (ε0, vacuumpermittivity; εr, relative permittivity; and E_(b), breakdown field(MV/cm) calculated by dividing breakdown voltage by film thickness).

The leakage current densities of all the nanocomposite films prepared inthis investigation (FIGS. 8 and 9) are mostly within the range 10⁻⁸-10⁻⁶A/cm² at 100 V, indicating that the aforementioned nanocomposites areall excellent insulators. As the nanoparticle loading increases, most ofthe nanocomposites exhibit lower leakage current densities, presumably aresult of modified charge transport and interruption of the crystallineconduction pathways within the composite structure. However, at thehighest nanoparticle loadings, the nanocomposites have the largestleakage current densities, simply because the weight percentages of thenanoparticles have reached the respective percolation thresholds.Increasing the relative permittivity of the nanocomposite by changingthe shape of the inclusion does not appear to compromise the goodinsulating properties of these composites.

The present measured breakdown strengths for some of the nanocompositesare invariably ˜3-6 MV/cm, indicating that metal oxide nanoparticleinclusion does not significantly depress the polymer dielectricbreakdown strength. Without limitation, in a well-dispersed nanoparticlecomposite, interfaces between the ceramic nanoparticles and polymerphases can create effective electron scatterers and trapping centers,thus reducing the breakdown probability. Moreover, well-dispersedceramic nanoparticles may block degradation tree growth and can increasethe long-term breakdown strength. Energy densities of the presentnanocomposites are estimated to be as high as 9.4 J/cm³, which rivals orexceeds those reported for conventional ceramic, polymer, and compositedielectrics.

A challenge in the preparation of inorganic metal oxide-polyolefinnanocomposites is the general phase incompatibility between inorganicpolar metal oxide inclusions and the non-polar organic host materials.For example, ferroelectric metal oxides are highly hydrophilic, whileisotactic polypropylene is highly hydrophobic. Simple admixing of thetwo constituents negligibly disrupts the extensive nanoparticleagglomeration nor affects the um-scale or larger phase separation, whichcan lead to local dielectric breakdown and degrade the nanocompositeelectrical properties. In contrast, the present in situ supportedmetallocene polymerization approach minimizes these deficiencies byachieving homogeneous nanoscale dispersion of the metal oxide phase:each individual nanoparticle is surrounded by polymer chains propagatingin situ from the surface-immobilized metallocenium catalyst centers, andthus offers improved dielectric properties (energy densities as high as9.4 J/cm³).

However, such nanocomposites can have very large contrasts in relativepermittivities between host and guest materials, leading to a largedisparity in the electric fields within the constituent phases, thuspreventing the realization of maximum energy densities for bothconstituents simultaneously. For representative BaTiO₃-polypropylenenanocomposites, however, the achieved energy density is as high as 9.4J/cm³ although the materials permittivity ratio approaches ˜1000:1.Without limitation to any one theory or mode of operation, the Al₂O₃(∈_(r)≈10) layer (thickness ˜1 nm, estimated from ICP-OES analysis)evolving from ambient exposure of the MAO coating, can act as adielectric buffer layer between the high permittivity BaTiO₃nanoparticles (∈_(r)≈2000) and low permittivity polypropylene(∈_(r)≈2.2).

The most common effective medium models for permittivity are derived forthe simple case of a spherical dielectric inclusion embedded in a sphereof the host material. However, most materials do not occur naturally asspheres, and therefore effective medium models for other shapes havealso been developed. (See, e.g., Brosseau, C.; Beroual, A.; Boudida, A.J. Appl. Phys. 2000, 88, 7278-7288; Green, N. G.; Jones, T. B. J. Phys.D. Appl. Phys. 2007, 40, 78-85.) Simple analytical solutions for theeffective permittivity (∈_(eff)) can be derived only for ellipsoids,whereas all other shapes require numerical solutions. Depolarizationfactors along each semi-axis of the ellipsoid (N_(x), N_(y), N_(z)),where N_(x)=N_(y)=N_(z)=1, can be used to estimate geometrical effects.The depolarization factors are calculated from integrals, e.g., eq. 3,where a_(x), a_(y), a_(z) are the semi-axes of the ellipsoid. Forspheres, all three

$\begin{matrix}{N_{x} = {\frac{a_{x}a_{y}a_{z}}{2}{\int_{0}^{\infty}{\frac{1}{\left( {s + a_{x}^{2}} \right)\sqrt{\left( {s + a_{x}^{2}} \right)\left( {s + a_{y}^{2}} \right)\left( {s + a_{z}^{2}} \right)}}\ {\mathbb{d}s}}}}} & (3)\end{matrix}$depolarization factors are equal (1/3, 1/3, 1/3), however, forellipsoids the depolarization factors are, 0, 1/2, 1/2, respectively andfor discs, 1, 0, 0, respectively. Since the dielectric energy is astationary functional of the electric field, the result is thatpermittivities arising from spherical inclusions are the lowest and anydeviation from the spherical shape results in an increase in theeffective permittivity of the mixture at the same volume fraction. Theseobservations prompted study of TiO₂-isotactic polypropylenenanocomposites with different inclusion shapes.

In FIG. 10, the calculated effective permittivities of thenanocomposites containing spherical inclusions to the nanocomposites arecompared with ellipsoidal inclusions. For the case of sphericalinclusions, the effective permittivities are calculated using theMaxwell-Garnett effective medium theory (eq. 4), and for the case ofellipsoidal inclusions,

$\begin{matrix}{ɛ_{eff} = {ɛ_{b}\frac{ɛ_{a} + {2\; ɛ_{b}} + {2\;{f_{a}\left( {ɛ_{a} - ɛ_{b}} \right)}}}{ɛ_{a} + {2\; ɛ_{b}} - {f_{a}\left( {ɛ_{a} - ɛ_{b}} \right)}}}} & (4) \\{ɛ_{eff} = {ɛ_{b} + {\frac{f_{a}}{3}\left( {ɛ_{a} - ɛ_{b}} \right){\sum\limits_{{j = x},y,z}^{\;}\;\frac{ɛ_{eff}}{ɛ_{eff} + {N_{j}\left( {ɛ_{a} - ɛ_{eff}} \right)}}}}}} & (5)\end{matrix}$the effective permittivities are calculated using the Polder-Van Santenformalism (eq. 5), where ∈_(a) is the relative permittivity of the TiO₂inclusions, ∈_(b) is the relative permittivity of isotacticpolypropylene, f_(a) is the volume fraction of TiO₂ in the polymer, andN_(j) is for the depolarization factors. (See, e,g., Busico, V.;Cipullo, R.; Monaco, G.; Vacatello, M. Macromolecules 1997, 30,6251-6263.) As expected, the effective medium theory predicts thatcomposites containing ellipsoidal inclusions will have larger effectivepermittivities at low volume loadings than composites containingspherical inclusions.

The experimental results are plotted in FIG. 11. Remarkably, theeffective permittivities for spherical inclusions remain constant over asmall range of volume fractions, exactly as the Maxwell-Garnett equationpredicts (FIG. 10). In marked contrast, the effective permittivity ofcomposites having inclusions with ellipsoidal shapes increases rapidlywith increasing inclusion volume fraction, which is again similar totrend predicted for ellipsoidal inclusions using eq. 5 (FIG. 10).

EXAMPLES OF THE INVENTION

Materials and Methods. All manipulations of air-sensitive materials wereperformed with rigorous exclusion of oxygen and moisture in flamedSchlenk-type glassware on a dual-manifold Schlenk line or interfaced toa high-vacuum line (10⁻⁵ Torr), or in a dinitrogen-filled VacuumAtmospheres glove box with a high capacity recirculator (<1 ppm O₂ andH₂O). Propylene (Matheson, polymerization grade) was purified by passagethrough a supported MnO oxygen-removal column and an activated Davison 4Å molecular sieve column. Toluene was dried using an activated aluminacolumn and Q-5 columns according to the method described in literature,and was additionally vacuum-transferred from Na/K alloy and stored inTeflon-valve sealed bulbs for polymerization manipulations. BaTiO₃ andTiO₂ nanoparticles were kindly provided by Prof. Fatih Dogan (Universityof Missouri, Rolla) and Prof. Thomas Shrout (Penn State University),respectively. ZrO₂ nanoparticles were purchased from Aldrich. Thereagents 3 mol % yttria-stabilized zirconia (TZ3Y) and 8 mol %yttria-stabilized zirconia (TZ8Y) nanoparticles were purchased fromTosoh, Inc. TiO₂ nanorods were purchased from Reade Advanced Materials,Riverside, R.I. All of the nanoparticles were dried on a high vacuumline (10⁻⁵ Torr) at 80° C. overnight to remove the surface-bound water,known to adversely affect the dielectric breakdown performance. Thedeuterated solvent 1,1,2,2-tetrachloroethane-d₂ was purchased fromCambridge Isotope Laboratories (≧99 atom % D) and used as received.Methylaluminoxane (MAO; Aldrich) was purified by removing all thevolatiles in vacuo from a 1.0 M solution in toluene. The reagentdichloro[rac-ethylenebisindenyl]zirconium (IV) (EBIZrCl₂) was purchasedfrom Aldrich and used as received. n⁺-Si wafers (rms roughness≈0.5 nm)were obtained from Montco Silicon Tech (Spring City, Pa.) and cleanedaccording to standard procedures. Aluminum substrates were purchasedfrom McMaster-Carr (Chicago, Ill.) and cleaned according to standardprocedures.

Physical and Analytical Measurements. NMR spectra were recorded on aVarian Innova 400 (FT 400 MHz, ¹H; 100 MHz, ¹³C) spectrometer. Chemicalshifts (δ) for ¹³C spectra were referenced using internal solventresonances and are reported relative to tetramethylsilane. ¹³C NMRassays of polymer microstructure were conducted in1,1,2,2-tetrachloroethane-d₂ containing 0.05 M Cr(acac)₃ at 130° C.Resonances were assigned according to the literature for stereoregularpolypropylenes. Elemental analyses were performed by Midwest Microlabs,LLC, Indianapolis, Ind. Inductively coupled plasma-optical emissionspectroscopy (ICP-OES) analyses were performed by GalbraithLaboratories, Inc., Knoxville, Tenn. The thickness of the dielectricfilm was measured with a Tencor P-10 step profilometer and used tocalculate the dielectric constant and breakdown strength of the sample.X-ray powder diffraction patterns were recorded on a Rigaku DMAX-Adiffractometer with Ni-filtered Cu Kα radiation (1.54184 Å). Pristineceramic nanoparticles and composite microstructures were examined with aFEI Quanta sFEG environmental scanning electron microscope with anaccelerating voltage of 30 kV. Transmission electron microscopy wasperformed on a Hitachi H-8100 TEM with an accelerating voltage of 200kV. Composite melting temperatures were measured on a TA Instruments2920 temperature modulated differential scanning calorimeter. Typically,ca. 10 mg samples were examined, and a ramp rate of 10° C./min was usedto measure the melting point. To erase thermal history effects, allsamples were subjected to two melt-freeze cycles. The data from thesecond melt-freeze cycle are presented here.

Electrical Measurements. Gold electrodes formetal-insulator-semiconductor (MIS) devices were vacuum-depositedthrough shadow masks at (3−4)×10⁻⁶ Torr (500 Å, 0.2-0.5 Å/s). Directcurrent MIS leakage current measurements were performed using Keithley6430 sub-femtoamp remote source meter and a Keithley 2400 source meterusing a locally written LABVIEW program and general purpose interfacebus communication. A digital capacitance meter (Model 3000, GLKInstruments, San Diego) was used for capacitance measurements. Allmeasurements were performed under ambient conditions. Dielectricbreakdown strength measurements were carried out with a high-voltageamplifier (TREK 30/20A, TREK, Inc., Medina, N.Y.), and the experimentalparameters were: ramp rate, 1,000 V/S; peak voltage, 30,000 V; ext.amplifier, 3,000; temperature, room temperature.

With reference to the following representative examples, any metal oxidecomponent, metallocene catalyst component, aluminoxane component andolefin monomer component of the sort described herein can be usedinterchangeably with any one of the other. Accordingly, the generalprocedures of examples 1-2 were used to prepare the range ofnanocomposites referenced in conjunction with the corresponding figures,such procedures as can be modified by those skilled in the art madeaware of this invention. As such, the embodiments of example 3 were alsoprepared using such procedures (corresponding figures and data notshown).

Example 1

Representative immobilization of Metallocene Catalysts on Metal OxideNanoparticles. In the glovebox, 2.0 g nanoparticles, 200 mg MAO, and 50mL dry toluene were loaded into a predried 100 mL Schlenk flask. Uponstirring, the mixture turned into a very fine slurry. The slurry wasnext subjected to alternating sonication and vigorous stirring for 2days with constant removal of evolving CH₄. Next, the nanoparticles werecollected by filtration and washed with fresh toluene (50 mL×4) toremove any residual MAO. Then, 200 mg metallocene catalyst was loaded inthe flask with 50 mL toluene. The color of the nanoparticles immediatelyturned purple. The slurry mixture was again subjected to alternatingsonication and vigorous stirring overnight. The nanoparticles were thencollected by filtration and washed with fresh toluene until the color ofthe toluene remained colorless. The nanoparticles were dried on thehigh-vacuum line overnight and stored in the glovebox at −40° C.

Example 2

Representative Synthesis of Nanocomposites via In Situ PropylenePolymerization. In the glovebox, a 250 mL round-bottom three-neck Mortonflask, which had been dried at 160° C. overnight and equipped with alarge magnetic stirring bar, was charged with 50 mL dry toluene, 200 mgfunctionalized nanoparticles, and 50 mg MAO. The assembled flask wasremoved from the glovebox and the contents were subjected to sonicationfor 30 min with vigorous stirring. The flask was then attached to a highvacuum line (10⁻⁵ Torr), freeze-pump-thaw degassed, equilibrated at thedesired reaction temperature using an external bath, and saturated with1.0 atm (pressure control using a mercury bubbler) of rigorouslypurified propylene while vigorously stirring. After a measured timeinterval, the polymerization was quenched by the addition of 5 mLmethanol, and the reaction mixture was then poured into 800 mL ofmethanol. The composite was allowed to fully precipitate overnight andwas then collected by filtration, washed with fresh methanol, and driedon the high vacuum line overnight to constant weight.

Example 3

More specifically, with reference to the preceeding, BaTiO₃ and TiO₂ 40nm nanoparticles were dried on a high-vacuum line to removesurface-bound water, known to adversely affect dielectric breakdownperformance. Nanocomposites were then synthesized via sequentialnanoparticle MAO functionalization, catalyst immobilization/activation,and in situ isotactic propylene polymerization (Schemes 1 and 2). Thefirst step is the anchoring MAO onto the nanoparticle surfaces viasurface hydroxyl group reaction to form covalent Al—O bonds. AnchoredMAO functions as a cocatalyst to activate the metallocene, and inaddition, the hydrophobic MAO helps disrupt, in combination withultrasonication, hydrophilic nanoparticle agglomeration in thehydrophobic reaction medium. After washing away unbound MAO, theMAO-coated nanoparticles are subjected to reaction with the C₂-symmetricpolymerization catalyst EBIZrCl₂ to afford surface-anchored,polymerization-active species. EBIZrCl₂ is known to produce highlyisotactic polypropylene, which, in conventional capacitors, affordsenhanced mechanical and dielectric properties at elevated operatingtemperatures. Subsequent in situ polymerization yields isotacticpolypropylene-BaTiO₃/TiO₂ nanocomposites, the compositions of which canbe tuned by the polymerization conditions.

As demonstrated, the present invention provides a range ofwell-dispersed metal oxide-polyolefin nanocomposites via a scalable, insitu supported metallocene olefin polymerization process. Leakagecurrent densities ˜10⁻⁸-10⁻⁶ A/cm² suggest that the nanocomposites areexcellent insulators. The relative permittivity of the nanocompositesincreases as the nanoparticle fraction increases. At the same inclusionloading, rod-shaped TiO₂ nanoparticle-polypropylene nanocompositesexhibit significantly greater relative permittivities than sphere-shapedTiO₂ nanoparticle-polypropylene nanocomposites. Energy densities of theBaTiO₃-polypropylene nanocomposites are found to be as high as 9.4J/cm³. This versatile approach offers effective control over compositecomposition and ready scalability. That is, simply by varyingnanoparticle identity as well as their sizes, shapes, and themetallocene catalysts used, a wide array of nanocomposites with desireddielectric and mechanical properties can thus be catalyticallysynthesized in situ.

1. A method of preparing a metal oxide-polyolefin nanocomposite, saidmethod comprising: providing a substrate comprising a nano-dimensionedmetal oxide component and a metallocene olefin polymerization catalystcomponent coupled thereto, and contacting said substrate with an olefincomponent, said contact for at least one of a time and in an amountsufficient to polymerize said olefin on said substrate, to provide saidnanocomposite wherein said metal oxide component is homogenouslydispersed throughout said nanocomposite such that said nanocomposite hasan energy density of from between 2-9.4 J/cm³.
 2. The method of claim 1wherein said metal oxide is selected from binary and ternary metaloxides and said oxides comprise a dopant; and said polyolefin componentis selected from C₂ to C₁₂ polyalkylenes, substituted C₂ to C₁₂polyalkylenes, and copolymers thereof.
 3. The method of claim 2 whereinsaid substrate has a volume percentage ranging from 0.05 percent to 15percent of the total nanocomposite.
 4. The method of claim 3 whereinincreasing said volume percent affects at least one of melt temperature,leak current density and relative permittivity of said nanocomposite. 5.The method of claim 1 wherein said metal oxide component has a shapeselected from spherical and rod-like.
 6. The method of claim 5 whereinincreasing relative permittivity of said nanocomposite is achieved byselecting a rod-shaped metal oxide component.
 7. A method of using analuminoxane component to moderate phase energy densities of a metaloxide-polyolefin nanocomposite, said method comprising: providing ametal oxide component, said metal oxide selected from binary and ternarymetal oxides and said oxides comprising a dopant; contacting said metaloxide component and an aluminoxane component for a time sufficient toprovide an aluminum oxide coating on said metal oxide component;contacting said coated metal oxide with a metallocene olefinpolymerization catalyst component, to provide a nano-dimensionedsubstrate; and contacting said substrate with an olefin component, saidcontact for at least one of a time and in an amount sufficient topolymerize said olefin on said substrate, wherein said metal oxide ishomogenously dispersed throughout said metal oxide-polyolefinnanocomposite such that said metal oxide-polyolefin nanocomposite has anenergy density of from between 2-9.4 J/cm³.